In our U.S. Pat. No. 7,152,001, the entirety of which is herein incorporated, there is disclosed a computer based system for predicting the fluid level in a fluid flow network. The system has been very successful as it can use past and present measurements of parameters to predict and control fluid level and flow. The system gathers data from timed fluid levels and opening positions of regulators or valves to provide a model from which fluid levels and flow can be determined in real time. An irrigation channel is an open hydraulic system that serves to convey water from a source supply to end customers. Along the channel, flows and water levels are regulated via control gates situated at discrete points. FIG. 1 of U.S. Pat. No. 7,152,001 shows a side view of a typical channel regulated by overshot gates. The stretch of channel between gates 16, 18 is referred to as a pool. Water flows under the power of gravity, from a water source along the channel to farms. In view of this, the water levels along the channel correspond to the potential energy available to produce flow of water along the channel itself, into lateral distribution systems and onto land to be irrigated. It is therefore important to maintain the water levels above the levels required to meet flow demand.
The goal of automating an irrigation channel is to improve distribution efficiency in terms of the water taken from the supply and the water delivered to end customers. This is achieved by employing advanced instrumentation and control systems of the type shown in U.S. Pat. No. 7,152,001, which provide for a closer match between the water ordered by the farmers and the volume of the water moving through or flowing into the channel system, while maintaining the water levels along the channel system within operational limits dictated by quality of service and safety concerns.
U.S. Pat. No. 7,152,001 includes sensors 24, 26, 28, 29 and actuators linked through a Supervisory Control and Data Acquisition (SCADA) communication network 44 and advanced control practices that work in conjunction with each other to achieve high distribution efficiency, reduce transmission losses and provide high level of service to the customer/farmer thereby yielding high productivity from water which is a limited resource. When a channel is fully automated, the channel control gates 16, 18 are operated in such a manner so as to meet the demand for water downstream of the control gates 16, 18 and to maintain the water level upstream of the gate or regulator in every pool. A certain level of water must be exceeded in each pool to provide the potential energy needed to propel water further downstream, into secondary channels and onto the adjacent farms. The volume of water flowing into the channel system is controlled at the upstream or top end. The volume of water flowing into the channel is increased if a drop in the water level is sensed in a given pool or is reduced if the water level rises ensuring a constant water level is maintained.
A reactive control strategy is employed to maintain the water level in pools at their set points, e.g. control action is taken only when the controlled variable (water level in a pool) deviates from its set point. This is often referred to as feedback control. Measured flow information at the downstream regulator 18 in a pool and at the lateral off takes and at the farm outlets (if available) can be exploited to augment the feedback controller and make the control system more responsive. Often referred to as feed forward control, the upstream gate 16 sends a percentage of the measured outflows immediately rather than waiting for the flows to affect the water level in the pool and the feedback controller to take action.
The reactive control architecture described above confines the propagation of transients to upstream of changes in flow load (i.e. an out flow starting or stopping). This has merit in terms of the corresponding demand driven release of water from the upstream source; i.e. water is released from the top only when there is an out flow due to an off take downstream and this is cut off when the off take stops. However, the achievable transient performance is fundamentally limited by inherent transport delays, particularly in terms of transient peaking of control gate flow commands and deviation in water levels in response to an increase in flow load and similar undesirable effects when flow load is reduced.
FIG. 1 of the drawings shows a graph of a flow peak amplification along a channel operated using U.S. Pat. No. 7,152,001 for a 55 Megaliter/day step up 20 in flow from bottom control gate in the channel. The first and main limitation of the control strategy depicted in FIG. 1 is the limited transient flow characteristic. The peaks in the transient flows commands for the control gates 16 are amplified as the effect of a load change propagates upstream. Transient behaviour as depicted in FIG. 1 can result in actuator (i.e. control gate) saturation, thereby triggering undesired behaviour. This is the second limitation of the existing strategy used in U.S. Pat. No. 7,152,001. The mechanism to counter saturation, often called anti-windup in the control industry, is designed as an afterthought in U.S. Pat. No. 7,152,001 and this may not be very effective. In the case of long pools e.g. greater than 5 km that have limited storage volume, the flow transient may result in unacceptable water level deviations that may affect service to customers/farmers or it may violate safe operational limits. This is the third limitation of the existing strategy. The third limitation means that the existing control strategy cannot be applied to open irrigation channels with very limited freeboards. This is the fourth limitation. “Freeboard” is the height of the channel bank above the highest water level anticipated.